Different render per layers?
I going to assume that you really mean 15 layers (objects in separate blender viewport layers) and not referring to render passes (Z, Vector, Normal). +someonewithpc is correct, but I thought I would expand and clarify the answer.

Here's what you need to do. I'm going to assume that you will want multiple render passes per render layer, since it's the more complicated solution. If not, it will be a little simpler.


View the image in a new window to see it full resolution.


Set your output file format to OpenEXR MultiLayer in the the render output panel.
In the Render Layers panel, setup a render layer for each of the viewport layers you want to render. Select the output render passes you need for each render layer (Z, normal, etc)
In the compositor, Add a Render Layer node for each render layer you setup.
Create a File Output node for each corresponding Render Layer node. In the property panel on the right, you will need to manually add additional output layers in the file output node (Z, alpha, etc). You can do this because you set the output file to OpenEXR Multilayer previously.
Hook up the matching input/outputs between each render layer node and file output node. You'll also have to enter the output Path in each file output node. It's good to choose a separate folder to keep each Render Layer frame set separated.
Make sure that the Compositing checkbox is checked in the Post Processing panel in the Render Property section.
Render the animation. You should end up with one folder per layer with 1 EXR file per frame rendered. In each EXR file you will have the render passes selected (Z, normal, alpha, etc.)
You can use the EXR files for further compositing in Blender or another external compositing app. 


Note: The main render image (whatever is piped to the composite node in the compositor will still be rendered and written out. I believe you still need to have this hooked up for the rest of the process to work properly (I haven't tested this recently but know it worked like that in the past).